docs(mobile-client): add features overview page

- Create new page to showcase key features of the Flutter News App Mobile Client
- Include CardGrid component to display individual feature guides
- Add descriptions and links for each feature: Headlines Feed, Headline Details, Entity Details, Search, Authentication, Account Management, and Settings

Author: fulleni

src/content/docs/mobile-client/features/index.mdx

@@ -0,0 +1,39 @@
+---
+title: Features
+description: Explore the key features of the Flutter News App Mobile Client.
+---
+
+import { Card, CardGrid } from '@astrojs/starlight/components';
+
+The mobile client is packed with features designed for a modern, engaging news application. Explore the guides below to understand its core capabilities.
+
+<CardGrid>
+  <Card title="Headlines Feed" icon="newspaper">
+    Learn how the main news feed is managed, including filtering and dynamic content.
+    [Read more &rarr;](/docs/mobile-client/features/headlines-feed/)
+  </Card>
+  <Card title="Headline Details" icon="article">
+    Learn how the mobile client displays individual articles and similar content.
+    [Read more &rarr;](/docs/mobile-client/features/headline-details/)
+  </Card>
+  <Card title="Entity Details" icon="document">
+    Learn how the app displays detailed pages for topics and sources.
+    [Read more &rarr;](/docs/mobile-client/features/entity-details/)
+  </Card>
+  <Card title="Search" icon="search">
+    Learn how the search feature allows users to find headlines, topics, and sources.
+    [Read more &rarr;](/docs/mobile-client/features/search/)
+  </Card>
+  <Card title="Authentication" icon="shield">
+    Learn how user authentication is handled in the mobile client.
+    [Read more &rarr;](/docs/mobile-client/features/authentication/)
+  </Card>
+  <Card title="Account Management" icon="user-cog">
+    Learn how users can manage their preferences, saved articles, and followed content.
+    [Read more &rarr;](/docs/mobile-client/features/account/)
+  </Card>
+  <Card title="Settings" icon="settings">
+    Learn how users can customize the application's appearance, language, and feed display.
+    [Read more &rarr;](/docs/mobile-client/features/settings/)
+  </Card>
+</CardGrid>